The present invention is directed to a heterodyne stage of a radio or pager receiver, and more particularly, to a closed-loop control circuit for use therein in controlling the bias current of a local oscillator within a banded range to render the injection signal thereof precisely at a desired amplitude.
A heterodyne stage of a radio or pager receiver includes a local oscillator which generates an injection signal at a predetermined frequency and at an amplitude that is proportional to the amount of bias current supplied to the oscillator, and a mixer circuit which is governed by the injection signal of the local oscillator to convert a mixer input signal at one frequency to a mixer output signal at another frequency based on the frequency of the injection signal. Normally, in a receiver heterodyne stage, the frequency of the injection signal is subtracted from the frequency of the input signal to the mixer to render an output signal of the mixer at a frequency substantially corresponding to the resulting frequency difference. A receiver of the aforementioned type may include one or more heterodyne stages for converting received radio frequency or RF signalling to an intermediate frequency or IF and possibly to convert one IF signal to another lower IF signal for utilization by further downstream circuitry.
Receivers of the portable variety are battery powered and thus considerable emphasis is placed on conserving power consumption by such receivers to extend the usable operating life thereof without having to recharge or replace its batteries. To this end, some more recent portable receiver units have included power conservation measures which are designed to de-energize certain circuits of the receiver when not needed and then re-energize them according to demand. It is always of some concern in the transition from de-energization to re-energization that each of the circuits restart and operate at designed performance levels in a short time interval.
For example, restarting a crystal-controlled local oscillator circuit requires approximately on the order of twice the bias current than what is needed after the oscillator circuit reaches steady state conditions. Accordingly, what is desired is to provide a large amount of bias current to the oscillator circuit initially upon re-energization and then adjust back to the minimum bias current needed to sustain the oscillator injection signal at a desired amplitude while operating under steady state conditions. The problem is that it is not always easy to estimate these two bias current level extremes under all working conditions. Selecting current levels that are too high may result in unpredictable oscillator performance, as well as excessive gain in the mixer stage and undesirable consumption of power from the battery which will decrease the operational battery life of the receiver. Selecting too low a level may cause the oscillator not to restart upon re-energization or to cause the amplitude of the injection signal to fall below operational limits, resulting in degraded mixer performance.
One solution to ensuring proper restart of the oscillator circuit and protecting against loss of the injection signal is to provide a closed feedback loop at the heterodyne stage to control the bias current to the oscillator circuit in accordance with a desired injection signal amplitude. Such a solution is proposed in U.S. Pat. No 3,805,162 issued Apr. 16, 1974 to Clive Hoffman et al. which patent being assigned to the same assignee as the instant application. The proposed circuit of the Hoffman et al. patent is directed to detecting an oscillator injection signal amplitude and setting a desired amplitude thereof based on the difference between base-emitter voltages of a mixer transistor having high frequency operational characteristics and a control stage transistor having lower frequency operational characteristics. While the proposed closed loop control circuitry of Hoffman et al. is considered adequate for many operational conditions, it is believed not without need of improvement.
Some factors which must always be considered in evaluating such types of feedback control loop circuits are: (1) the ability to measure the amplitude of the injection signal undistorted by extraneous signals such as broadband RF signalling provided at a first heterodyne stage operational at the input of such a receiver unit, (2) the ability to set a precise reference level for controlling the amplitude of the injection signal to the mixer such that it does not deviate substantially during the heterodyne operation of the mixer circuit, and (3) to ensure that the closed-loop control circuit operates within a bounded bias current range to sustain oscillator operation even under extreme abnormal operating conditions.
In the present invention, these and other factors are provided for in a closed-loop control circuit for controlling the oscillator injection signal to a desired amplitude by adjusting the bias current to the oscillator circuit in a heterodyne stage. The advantages of Applicants' invention over the prior art will become more evident from the following description of preferred embodiments and accompanying drawings.